In flat panel displays (e.g., backlight computer displays), optical film (which can also be referred to as a sheet, layer, foil, and the like) materials are commonly used, for example, to direct, diffuse, or polarize light. For example, in backlight displays, brightness enhancement films collimate light using prismatic structures on the surfaces thereof to direct the light along a viewing axis (i.e., an axis normal (perpendicular) to the display). This enhances the brightness of the light viewed by the user of the display and allows the system to consume less power in creating a desired level of on-axis illumination. Such films can also be used in a wide range of other optical designs, such as in projection displays, traffic signals, and illuminated signs.
Brightness enhancing display films generally termed “prism sheets” comprises a series of prisms disposed upon a film. FIG. 1 is an exemplary depiction of the series of prisms disposed upon a film. Each brightness enhancing display film comprises a prism surface and a back surface. The prism surface of the brightness enhancing display film is the surface that comprises the upper surfaces (the air contacting surfaces) of the prisms. The back surface of the brightness enhancing display film is the surface opposed to the prism surface. The back surface is generally flat and is parallel to the base of the prisms that are disposed upon the film substrate as shown in the FIG. 1. It is desirable for the brightness enhancing display film to collimate most of the light incident upon the back surface of the film substrate in the on-axis direction. As can be seen in the FIG. 1, the on-axis brightness is the brightness measured in a direction perpendicular to the back surface of the brightness enhancing display film.
Currently, backlight displays, for example, employ a plurality of films arranged in a manner to obtain the desired brightness and diffusion of the light directed to the viewer. It is noted that as the number of films employed increases, the overall thickness of the backlight display increases. It is noted, however, that consumers are increasingly demanding thinner backlight display devices. Moreover, it is also desirable to eliminate color bands that may be observable in the back light display device to further meet consumer demands. If the film changes the polarization state of the light differently point-to-point, then that light passes through the polarizing films differently point-to-point such that the viewer sees color-bands or stripes of different color (e.g., shades of grey).
Since a demand exists for increasingly thinner backlight display devices, what is needed in the art is a multifunctional brightness enhancement film with no visible color bands. Color bands manifest as patterned variations in brightness or color of the final display and are often seen best when the display is viewed at a glancing angle. These color effects are often the results of stresses in the films that make up the display. When there are stresses in a plastic film, the plastic molecules tend to orient. The most common way of quantifying these types of stresses utilizes the fact that light polarized in the direction of molecular orientation travels through the plastic at a slightly different speed then light that is polarized perpendicular to the molecular orientation. The retardation of the slower light orientation relative to the faster light orientation can be quantified as the distance by which the wave phase has shifted. We refer to this measurement as the stress retardation.